The present invention relates to a check valve for use in a fluid producing well. More particularly the invention relates to a retrievable check valve which provides simple means for equalization of pressures above and below the valve.
Check valves are commonly used within a string of tubing of a fluid producing well wherein a submersible pump is being used to move fluid to the surface. The pump is usually located at the fluid bearing formation, adjacent the foot of the string of tubing. Usually impellors within the pump are electrically driven to force the fluid upwardly. Thus, while the pump is in operation, a column of fluid extends the length of the tubing string above the pump. In the event of stopping the pump, this suspended column, if not restricted, would back flow through the pump. This back flow would reverse the impellors at high speed, thereby causing damage to the thrust bearings. Therefore a check valve is placed in the tubing string above the pump to regulate the back flow of fluid through the pump.
Many such check valves are in common use, the simplest of which is a ball-in-a-cage valve. Here a tubular cage is provided having a seat at its lower end. A ball within the cage seats on the seat and is dislodged in response to the upward flow of fluids. Back flow however is restricted as the ball becomes reseated. Such valves suffer the disadvantage known as "squirrel caging", wherein the gaseous nature of the fluid flow causes the ball to bounce around in the cage, causing damage to both the ball and cage, and ultimately resulting in a loss of effective seal.
Flapper check valves are also known in the art. According to this design, a horizontal flap in a tubular body is hinged to open and close in response to flow pressures bearing from above and below the valve. In the past, both the flapper and the ball-in-a-cage type valves have had the tubular body threaded into the tubing string, making the valves non-retrievable by design. Thus to replace or repair the valve, it was necessary to pull out the entire tubing string, a time-consuming and costly process.
A further type of check valve used in well strings is the dart-type valve. A dart is seated in the lower portion of a closed tubular body and is provided with an upper tubular shaft. A narrow upper neck portion of the tubular body guides the dart shaft as the dart is translated upwardly to allow flow past the valve. Valves of this type are generally subject to jamming problems, since considerable sand tends to lodge itself in the upper tubular body above the dart shaft, restricting the upward movement of the dart.
These previously described check valves commonly suffer the disadvantage of not providing simple means to equalize pressures above and below the valve. Once the valve has closed to restrict back flow of the fluid therethrough, it is desirable to allow the column of fluid suspended above the valve to drain slowly past the valve to thereby equalize pressures without damaging the thrust bearings. Otherwise, to remove the valve or pump, the tubing string, laden with the fluid column, must be pulled to the surface. In deep wells, the additional weight of the fluid column is considerable, making the pulling operation difficult.
Heretofore, to provide for equalization in check valves has been a complicated matter of providing by-pass ducts which open and close either by surface control or in response to changing pressures bearing on the valve. For instance, in U.S. Pat. No. 2,994,280 to Daffin, there is disclosed a retrievable check valve of a ball-in-a-cage design which is provided with equalization means. Shear pins are used to hold an outer sleeve of the tubular body in place. An upward pull on the valve shears the pins, allowing the sleeve to translate upwardly, thereby aligning bypass ports of the sleeve and inner tubular body to allow fluid to flow therethrough.